1. Field of the Invention
The present invention relates to the power factor which loads present to AC power lines generally and more specifically to the reduction of the harmonic currents generated on an AC power line by a DC power supply.
2. Description of the Prior Art
The power factor that a load presents to an AC power line has long been of concern. At one time, the primary concern was the inductive component of some loads. Consider, for example, the section taken from the text entitled American Electricians' Handbook (8th ed. 1961) which was edited by Terrell Croft, revised by Clifford C. Carr, and published by McGraw Hill Inc.
"143. Correction of Low Power Factor. In industrial plants, excessively low power factor is usually due to underloaded induction motors because the power factor of motors is much less at partial loads than at full load. Where motors are underloaded new motors of small capacity should be substituted. Power factor can be corrected (1) by installing synchronous motors (2) by connecting static capacitors across the line." (end of section) PA0 "6.2.7 Harmonic current. The operation of equipment shall be designed to have minimum harmonic distortion effect on the electrical system. The operation of such equipment with the following specified ratings shall not cause harmonic line currents to be generated that are greater than 3 percent of the unit's full load fundamental current between the 2nd and 32nd harmonic. . . . " PA0 "Additionally, currents with frequencies from the 32nd harmonic through 20 kilohertz (kHz) shall not exceed 100/n percent . . . " PA0 " . . . the existance of harmonics in the line makes it not only impractical, but also impossible to filter out all of the harmonic current. An attempt to do so may result in overloading the filters. Accordingly, it has been determined that the better approach is to remove only a percentage of the harmonic currents generated as a result of the load and control device. Thus, a certain amount of harmonic current is reflected back into the supply line. The amount of harmonic current so reflected can be adjusted depending upon the requirements of the utility system supplying power." PA0 "(i)f the utility requirements on harmonics are more severe, it is possible to remove an even higher percentage such as 75%."
Of late, the "power factor" presented by DC power supplies has become a concern. Typically, DC power supplies employ a bridge rectifier, a filter capacitor, and, sometimes, a filter choke. The input of the rectifier is coupled (by a fuse, switch, etc.) across an AC power line. The output of the rectifier is either coupled by the choke across the capacitor (choke input filter) or, absent the choke, directly connected across the capacitor (capacitor input filter) to develop a DC (output) potential across the capacitor.
With the choke (input filter), DC power supplies draw from the AC power line a current the waveform of which approximates a square wave (when the inductance of the choke is much greater than what is commonly referred to as the "critical" inductance). Absent the choke, the waveform more approximates a series of pulses each of which is synchronized with a corresponding peak of the AC power-line potential. In either case, the current drawn from the AC power line includes harmonic components (currents), one for each of the odd harmonics of the AC power-line frequency. DC power supplies do not conform to all of the old power factor conventions. (For example, it makes little sense to define the power factor a DC power supply presents to an AC power line as the cosine of the phase angle between the voltage developed across the input of the DC power supply and the current flowing into it.) However, they (DC power supplies) do present many of the same problems. DC power supplies, like other loads having a relatively low power factor, draw from the AC power line a current the rms level of which is disproportionately high in relation to the current that should be drawn for the power consumed. (In other words, they do conform to the definition which states that the power factor of a load (in this case, a DC power supply) is given by the ratio of the actual power consumed (in this case by a load connected to the output of the DC power supply) (as indicated by a wattmeter) to the apparent power (as indicated by the combination of a (true rms, iron-vane or thermocouple-type) ammeter and a voltmeter) (connected to the input of the DC power supply).
A relatively high AC power-line rms current is of concern in that the AC power-generating facilities and AC power-transmission facilities (lines and transformers) must be sized to accommodate the current. Further, generation and transmission losses are primarily resistive losses which, therefore, increase as the square of the level of the rms AC power-line current. It is important to note that even relatively small loads (DC power supplies) may be of concern. Although a small personal computer, for example, may not draw the level of the current drawn by a large smoke stack scrubber, if the DC power supply of the computer has a relatively low power factor, the current drawn by the DC power supply may be of such a level as to limit what may also be plugged into a single AC power-line wall outlet.
In addition, DC power supplies present many special problems, particulary for the military. As the number of airborne and shipborne systems employing DC power supplies has increased, so has the level of harmonic currents generated on the various AC power lines. With the increased levels of harmonic currents has come an increase in the above mentioned generation and transmission problems. In addition, and of potentially much more serious consequence, the high levels of harmonic currents are causing problems of undesired system interaction. As a consequence, standards have been promulgated including those in the Department of Defense document which is identified as DOD-STD-1399(NAVY) SECTION 300 Aug. 1, 1978 and which is entitled MILITARY STANDARD INTERFACE STANDARD FOR SHIPBOARD SYSTEMS SECTION 300 ELECTRIC POWER, ALTERNATING CURRENT. Of particular relevance is the following section.
(For a power-source frequency of 60 Hz, a 1 kVA or more unit rating is specified.)
In Table I. (on page 5) the total harmonic distortion is limited to 5 percent; and, in section 6.2.2 the power factor is limited to the range of 0.8 lagging to 0.95 leading. Further, in Table I. (on page 5) the worst case frequency excursion from nominal frequency is listed as 51/2 percent.
Heretofore, great difficulty has been had in meeting the above-mentioned standard.
A prior-art-type circuit for improving the power factor a DC power supply presents to an AC power line is shown in the German patent number DE 3012-747 and the Japanese patent number 58-163271. Both circuits include an inductor and a capacitor which is connected in parallel with the inductor. The inductor-capacitor combination is connected between the AC power line and the input of the bridge rectifier of a (capacitor-input-filter-type) DC power supply to couple the DC power supply to the AC power line. In the German patent it is indicated that the inductor-capacitor combination is usually tuned to the fifth but maybe the ninth or thirteenth harmonic of the AC power-line frequency. And, in the Japanese patent it is indicated that the purpose of the inductor-capacitor combination is to reduce the level of the third and fifth harmonic waves (currents?).
Although of some value in reducing the level of some of the harmonic currents, it is important to note that the reductions afforded by the above-mentioned inductor-capacitor combination does not approach that required to meet the above mentioned standard. Further, while reducing the level of certain harmonic currents, the above-mentioned inductor-capacitor combination may increase the level of other harmonic currents. Finally, The use of the above mentioned inductor-capacitor combination may reduce the level of the DC output potential developed by the associated DC power supply.
Disclosed in the British patent 1,472,411 of T. Kennedy is a filter network which is for use with a load having a non-linear control device (saturable reactor) and which is for absorbing unwanted harmonic currents. The filter network employs a plurality of filters each including an inductor and a capacitor which is connected in series with the (associated) inductor. Each of the filters (inductor-capacitor combinations) is connected in parallel with the load. An additional inductor is employed connected between the AC power line and the load to couple the load to the AC power line. It is indicated (on page 2 in lines 113-115 of the British patent) that each of the filters (inductor-capacitor combinations) is tuned to a frequency less than the harmonic frequency which it is to filter. Further, it is indicated (on page 3 in lines 115-128) that
In an example in the British patent it is indicated (on page 4 in lines 109-112) that 70 percent of the harmonic currents are removed and 30 percent of the harmonic currents are reflected into the AC power line. Further, it is indicated (on page 4 in lines 112-115) that
Again, although of some value in reducing the level of some of the harmonic currents, it is important to note that the reductions in the levels of harmonic currents afforded by the network disclosed in the above-mentioned British patent does not approach that required to meet the above mentioned standard.
Finally, in the U.S. Pat. No. 4,222,096 of D. Capewell and the U.S. Pat. No. 4,369,490 of F. Blum a circuit is disclosed which includes a capacitor connected in parallel with the input of the bridge rectfier of a (capacitor-input-type) DC power supply and an inductor connected between the AC power line and the input of the rectifier to couple the DC power supply to the AC power line. In the F. Blum patent it is indicated (on column 5 in lines 23-29) that without the above-mentioned circuit, the DC power supply was found to present a power factor of 65 percent to the AC power line. Also, it was found that without the above-mentioned circuit, the level of the third harmonic current was 88 percent, the level of the fifth harmonic current was 65 percent, and the level of the seventh harmonic current was 38 percent of the level of the fundamental current. In one example, with the above-mentioned circuit, the DC power supply was found to present a power factor of 94 percent to the AC power line. Also, with the above-mentioned circuit, the level of the third harmonic current was 20 percent, the level of the fifth harmonic current was 6 percent, and the level of the seventh harmonic current was 2 percent of the level of the fundamental current.
Although the above-mentioned circuit greatly increases the power factor a DC power supply presents to an AC power line and greatly reduces the levels of the harmonic currents. It is important to note that the DC power supply (and circuit combination) still does not even come close to meeting the above-mentioned military standard.